Miniature board-to-board electrical connector assemblies are known in the art. Conventionally, a first or receptacle connector is mounted on a first printed circuit board, and a plug or second connector is mounted on a second printed circuit board. The plug connector is inserted into the receptacle connector, with the two printed circuit boards in generally parallel relationship.
FIGS. 1-3 show such an assembly as is known in the prior art. Specifically, a board-to-board electrical connector assembly, generally designated 14, includes a first connector, generally designated 16, mounted on a first printed circuit board 18, and a second connector, generally designated 20, mounted on a second printed circuit board 22. First connector 16 mounts a plurality of conductive terminals, generally designated 24, and second connector 20 mounts a plurality of conductive terminals, generally designated 26. The terminals are in two parallel rows.
More particularly, first connector 16 includes a dielectric housing, generally designated 28, which is generally rectangular and has opposite, elongated side walls 28a, an elongated central platform 28b and an insertion groove, generally designated 28c, between the side walls and the central platform. Each terminal 24 includes a mounting post 24a fixed within a respective one of the side walls 28a of the housing, along with a tail portion 24b connected, as by soldering, to an appropriate circuit trace on the first printed circuit board 18. Each terminal includes a generally U-shaped contact portion defined by a base section 24c and to side sections 24d and 24e joined to the base section. Side section 24b has a contact portion 24f at the distal end thereof, and side section 24e has a retaining portion 24g at the distal end thereof. The U-shaped contact sections are disposed at the bottom of insertion groove 28c of housing 28.
Second connector 20 includes a dielectric housing, generally designated 30, which includes a bottom wall 30a and pair of side walls 30b defining a receiving groove, generally designated 30c, between the side walls. Terminals 26 are mounted on the side walls to define two rows of terminals for mating with the two rows of terminals 24 of the first connector 16. Each terminal 26 includes a body portion 26a fixed within a respective one of the side walls 30b of the housing, along with a tail portion 26b for connection, as by soldering, to an appropriate circuit trace on the second printed circuit board 22. Each terminal includes a contact portion 26c at the inside surface of the respective side wall 30b. 
When connectors 16 and 20 are mated, side walls 30b of housing 30 (along with contact portions 26c of terminals 26) of connector 20 move into insertion groove 28c of housing 28 of connector 16. Simultaneously, platform 28b of housing 28 of the first connector moves into receiving groove 30c of housing 30 of connector 20. During mating, contact portions 24f of terminals 24 of the first connector engage contact portions 26c of terminals 26 of the second connector. Retaining portions 24g of terminals 24 rigidly abut against the outside surfaces of side walls 30b of housing 30 of the second connector, while contact portions 24f are free to flex within contact grooves 32 at opposite sides of platform 28b of housing 28 of the first connector.
In high frequency transmissions, the magnitude of contact impedance affects the transmission rate. With each terminal 24 having only one contact portion 24f engaging contact portion 26c of the respective terminal 26, there is only one contact point between each pair of mating terminals. Therefore, the contact impedance cannot be reduced effectively, resulting in a limited transmission rate. Consequently, the applicability of the board-to-board connector assembly 14 is reduced. The present invention is directed to solving these problems by providing contacts which effectively reduce the contact impedance in order to promote the transmission rate of electrical connector 16.